The progressive miniaturisation in almost all areas of microelectronics and microsystem technology requires constant development of all technologies with the aid of which the density of all kinds of functional units on substrates can be increased. These functional units include for example microcontrollers, memory chips, MEMS, all kinds of sensors or microfluidic components.
The technologies for increasing the lateral density of these functional units have been greatly improved in recent years. In some sub-sectors of microelectronics or microsystem technology, even to an extent such that a further increase in the lateral density of the functional units is no longer possible. In microchip production, the maximum achievable resolution limit for the structures to be produced lithographically has as good as already been reached. In a few years, therefore, physical and technological limitations will no longer permit any increase at all in the lateral density of functional units. For a number of years now, the industry has confronted this problem by the development of 2.5D and 3D technologies. With the aid of these technologies it is possible for identical, or even differently formed functional units to be aligned with one another, to be stacked upon one another, to bond them permanently with one another and to cross-link them with one another by suitable strip conductors.
One of the key technologies for producing such structures is permanent bonding. Permanent bonding is understood to mean all methods with the aid of which substrates can be bonded together in such a way that their separation is possible only by a high expenditure of energy and an associated destruction of the substrates. There are various kinds of permanent bonding, such as are known to the person skilled in the art.
One of the most important permanent bonding methods is fusion bonding, also referred to as direct bonding or molecular bonding. Fusion bonding is understood to mean the two-stage process of permanently bonding two substrates. In the first stage, also referred to as the pre-bond, the substrates are fixed together by relatively weak atomic forces such as the van der Waals forces and in the second stage are bonded together in a molecular and/or atomic manner by the formation of covalent bonds. Fusion bonds arise mainly at the surfaces of non-metallic inorganic materials.
The bonding strength created by the pre-bond suffices however to transport the two substrates without a displacement of the substrates with respect to one another being caused. Thus, although the bonding strength between the two substrates is certainly sufficient to transport the substrates stack without problem, the bonding strength is so small that a renewed, destruction-free separation of the two substrates can take place with special devices. This has the decisive advantage that, after a pre-bond, the structures of these two structures can be measured and their relative positions, distortions and orientations can be determined. If it is ascertained during the measuring procedure that a misorientation and/or a local and/or total distortion of the structures is present or particles are present in the interface, the substrate stack can again be duly separated and reprocessed. Following a successful and above all verified pre-bond, the actual permanent bond is produced by heat treatment processes. During the treatment process, a chemical and/or physical strengthening of the bonding of the surfaces of the two substrates arises due to the supply of the thermal energy. This permanent bond is irreversible in the sense that a destruction-free separation of the two substrates is no longer possible. Mention will only be made generally of a bond in the subsequent text and an explicit distinction between pre-bond and permanent bond will no longer be made.
The most common fusion bonds are carried out on silicon and silicon oxide substrates. As a result of its semiconductor properties, silicon often serves as a base material for the production of microelectronic components such as microchips and memories. A so-called direct bond can also arise between highly polished or lapped metal surfaces or with flat glass surfaces. The underlying bonding properties indeed differ from those of a fusion bond, but the mechanism by which the two surfaces make contact with one another by the progressive bonding wave can however be described by the same physics. The bonding of two hybrid surfaces by a so-called hybrid bond would also be conceivable. A hybrid surface is understood to mean a surface comprising at least two different materials. One of the two materials is usually limited to a small space, while the second material surrounds the first material. For example, metal contacts are surrounded by dielectrics. In the production of a hybrid bond by the bonding of two hybrid surfaces, the bonding wave is driven primarily by the fusion bonding between the dielectrics, whereas the metal contacts automatically find themselves together as a result of the bonding wave. Examples of dielectrics and low-k materials are                Non-silicon-based                    Polymers                            Polyimides                Aromatic polymers                Parylene                PTFE                                    Amorphous carbon                        Silicon-based                    Silicate-based                            TEOS (tetraethyl orthosilicate)                SiOF                SiOCH                Glasses (borosilicate glasses, alumosilicate glasses, lead silicate glasses, alkali silicate glasses, etc.)                                    General                            Si3N4                 SiC                SiO2                 SiCN                                    Silesquioxanes                            HSSQ                MSSQ                                                
One of the greatest technical problems with the permanent bonding of two substrates is the alignment accuracy of the functional units between the individual substrates. Although the substrates can be precisely aligned with respect to one another by means of alignment equipment, distortions of the substrates can arise during the bonding process itself. As a result of the distortions thus arising, the functional units will not necessarily be correctly aligned with one another at all positions. The alignment inaccuracy at a specific point on the substrate may be a result of a distortion, a scaling error, a lens error (magnification or reduction error) etc. In the semiconductor industry, all subject areas dealing with such problems are combined under the term “overlay”. A suitable introduction to this subject can be found for example in: Mack, Chris. Fundamental Physics of Optical Lithography—The Science of Microfabrication. WILEY, 2007, Reprint 2012.
Each functional unit is usually designed in the computer before the actual production process. For example, strip conductors, microchips, MEMS, or any other structure producible with the aid of microsystem technology, are designed in a CAD (Computer aided Design) program. During the production of the functional units, it can however be seen that there is always a deviation between the ideal functional units designed on the computer and the real functional units produced in the clean room. The differences are primarily due to natural variations in the materials used, such as for example different number of the different isotopes in the substrate material, limitations of the hardware, i.e. engineering-related problems, but very often physical limitations. Thus, the resolution accuracy of a structure that is produced by a photolithographic process is limited by the size of the apertures of the photomask and the wavelength of the light used. Mask distortions are directly transferred to the photoresist. Drives, (linear) motors and positioning devices of machines produced with these components can only approach reproducible positions within a given tolerance, etc. It is not therefore surprising that the functional units of a substrate cannot be identical to the structures designed on the computer. Even before the bonding process, all substrates thus have a non-negligible divergence from the ideal state.
If the positions and/or shapes of two opposite-lying functional units of two substrates are compared on the assumption that neither of the two substrates is distorted by a bonding process, it is found that in general there is already an imperfect congruence of the functional units, since the latter diverge from the ideal computer model due to the errors described above. The most frequent errors are represented in figure XX (Copied from: http://commons.wikimedia.org/wiki/File:Overlay-_typical_model_terms_DE.svg, 24 May 2013 and Mack, Chris. Fundamental Principles of Optical Lithography—The Science of Microfabrication. Chichester: WILEY, p. 312, 2007, Reprint 2012). According to the illustrations, a rough distinction can be made between overall and local as well as symmetrical and asymmetrical overlay errors. An overall overlay error is homogeneous, therefore independent of location. It produces the same divergence between two opposite-lying functional units irrespective of the position. The conventional overall overlay errors are errors I. and II., which arise due to a translation or rotation of the two substrates with respect to one another. The translation or rotation of the two substrates produces a corresponding translational or rotational error for all the functional units lying respectively opposite one another on the substrates. A local overlay error arises in a location-dependent manner, mainly due to elasticity and/or plasticity problems, in the present case primarily caused by the continuously propagating bonding wave. Of the represented overlay errors, errors III. and IV. are in particular referred to as “run-out” errors. This error arises primarily due to a distortion of at least one substrate during a bonding process. As a result of the distortion of at least one substrate, the functional units of the first substrate are also distorted in respect of the functional units of the second substrate. Errors I. and II. can however also arise due to a bonding process, but they are usually superimposed by errors III. and IV. to such a marked extent that it is difficult to detect or measure them.
There is already a device in the prior art, with the aid of which local distortions can be reduced at least partially. It concerns here a local distortion due to the use of active control elements (WO2012/083978A1).
Initial approaches to a solution for correcting “run-out” errors exist in the prior art. US20120077329A1 describes a method for obtaining a desired alignment accuracy between the functional units of two substrates during and after the bonding, whereby the lower substrate is not fixed. The lower substrate is thus not subjected to any boundary conditions and can bond freely to the upper substrate during the bonding process. An important feature in the prior art is, in particular, the flat fixing of a substrate, usually by means of a vacuum device.
In most cases, the arising “run-out” errors become more intensified radially symmetrical around the contact point, from which reason they increase from the contact point to the periphery. In most cases, it involves a linearly increasing intensification of the “run-out” errors. Under special conditions, the “run-out” errors can also increase non-linearly.
Under particularly optimum conditions, the “run-out” errors can be ascertained not only by suitable measuring devices (EP2463892), but can also be described by mathematical functions. Since the “run-out” errors represent translations and/or rotations and/or scaling between well-defined points, they are preferably described by vector functions. Generally, this vector function is a function f:R2→R2, i.e. a mapping rule, which maps the two-dimensional definition range of the position coordinates onto the two-dimensional value range of “run-out” vectors. Although an exact mathematical analysis of the corresponding vector fields has not yet been able to be carried out, assumptions are made concerning the function properties. The vector functions are, with a high degree of probability, at least Cn n>=1 functions, i.e. at least continuously differentiable. Since the “run-out” errors increase from the contact point to the edge, the divergence of the vector function will probably be different from zero. With a high degree of probability, therefore, the vector field is a source field.